I would never cut the dose nor would I play with the frequency of dosing. I have seen what happens to heartworm positive pups.
As for referring to HW preventatives as poison, I just lost one of my pups at an approximate age of 16. I adopted her in 1995 and she was estimated to be 2 or 3 at that time. She was given Heartgard every month while I had her...surely it did not shorten her lifespan.
The following website is very informative and may help some of you in your decision making regarding the health of your pup and the risks involved with heartworms:
American Heartworm Society